1. Field of the Invention
This invention relates to apparatus for automatically aligning objects. In particular, this invention relates to opto-mechanical apparatus for automatically aligning patterned silicon wafers and the like placed on a movable stage.
2. Description of the Prior Art
Integrated circuit chips or dice are manufactured by etching a plurality of circuits on a single large, substantially round substrate wafer made of semiconductor material. The plurality of circuits, each of which will later form a chip, are arrayed on the wafer in a rectalinear fashion with "streets" between adjacent circuits. At various points in the manufacture of the chips, the uncut partially finished wafers must be precisely aligned for probing, testing and the like. If this alignment process is automated, more wafers can be tested in a given period of time, and if the alignment is routinely more accurate, the yield of usable chips can be improved.
The field of pattern recognition has wide applications in industry. With the advent of photoelectric devices capable of generating electrical signals in response to optically detected objects, it has become possible to generate automatic devices for industrial use that detect and recognize objects.
One particular application for pattern recognition is in instruments that optically detect particular circuit elements or flaws in the production and quality control of photomasks and patterned silicon wafers. Two such instruments are shown in U.S. Pat. Nos. 4,347,001, and 4,247,203 both issued to Levy, et al., and relate to automatic photomask inspection systems and apparatus. Silicon wafers and the photomasks used to make silicon wafers generally have the same pattern repeated many times in a rectilinear array. The automatic photomask inspection system compares equivalent portions of two different dice on one photomask in two simultaneous scans. One problem with an apparatus like this is aligning the photomask or patterned silicon wafer precisely so that the two optical inspection paths are in fact inspecting equivalent portions of two different dice. A similar problem exists in wafer probers that perform electrical tests on wafers. Wafer probers employ fine conductive probes making contact with specific points on dice. This procedure demands extremely acurate alignment of wafers.
U.S. Pat. No. 4,200,861 and its divisional application, now U.S. Pat. No. 4,385,322, both issued to Hubach et al, teach a method and system for recognizing patterns. The location coordinates in the X and Y dimension for a match is determined at the location of the highest correlation number. A fine search is then made around the course location by comparing previously stored information in real time with information from the scene under investigation. No specific optics are given, and there is no suggestion of two separate low resolution data gathering scans at widely separated locations.
U.S. Pat. No. 4,300,164, issued to Sacks, teaches a pattern recognition system. A high contrast video signal is generated and patterns are recognized as combinations of black and white. Again, although this patent teaches comparison techniques and software, there is no specific teaching of the preferred optics.